Process for the elaboration of chrome steels

ABSTRACT

Chrome steels, particularly stainless steels, are made by blowing an oxidising gas into an electric furnace containing a bath of molten metal containing chromium to decarburise the metal, the oxidising gas being blown in below the surface of the metal and passing through the bath from the bottom upwards. The or each jet of oxidising gas is peripherally surrounded at its point of entry into the metal by a fluid refractory-protectant such as fuel oil. The metal bath initially contains more than 0.7% carbon, and can contain more than 1.2% carbon. The absence of any upper limit on the initial carbon content allows the use of carburised ferro-alloys, and the refining of relatively high carbon content baths.

United States Patent Leroy et al.

[ Dec. 30, 1975 3,708,599 1/1973 Kranse 13/9 3,751,242 8/1973 Knuppel.... 3,773,496 11/1973 Knuppel .1 75/60 Primary Examiner-Peter D. Rosenberg Attorney, Agent, or FirmCameron, Kerkam, Sutton, Stowell & Stowell [57] ABSTRACT Chrome steels, particularly stainless steels, are made by blowing an oxidising gas into an electric furnace containing a bath of molten metal containing chromium to decarburise the metal, the oxidising gas being blown in below the surface of the metal and passing through the bath from the bottom upwards. The or each jet of oxidising gas is peripherally surrounded at its point of entry into the metal by a fluid refractoryprotectant such as fuel oil. The metal bath initially ,contains more than 0.7% carbon, and can contain more than 1.2% carbon. The absence of any upper limit on the initial carbon content allows the use of carburised ferro-alloys, and the refining of relatively high carbon content baths.

12 Claims, No Drawings PROCESS FOR THE ELABORATION OF CHROME STEELS This invention concerns the production of chrome steels and particularly, but not exclusively, stainless steels.

It has long been known to produce stainless steels in an electric furnace by decarburizing the metal bath, with limited oxidation of the chromium, by blowing in pure oxygen through lances while bringing the bath to a high temperature. After decarburization, reduction of the oxides of chromium in the slag is effected by addition of, for example, ferrosilicon or silicochrome. In such a method, the resulting carbon content is generally less than 1 percent and cannot, without disadvantage, be higher than 1.2 percent. The actual speed of decarburization is generally below the maximum of between 1.5 and 2 percent of carbon per hour.

In this conventional procedure, the oxygen blown in by lance reacts essentially only with the surface layer of the metal bath which accordingly is rapidly and greatly decarburized whilst the overlying slag becomes very rich in oxides of iron and of chromium. The deeper part of the bath, however, is hardly if at all, decarburized. A considerable imbalance is thus established between, on the one hand, the surface layer of the bath and the slag, both highly oxidized, and on the other hand, the deeper part of the bath which remains rich in carbon. Experience has shown that if the carbon content of the deeper part is greater than about 1.2 percent, a very violent reaction usually occurs between this carbon and the oxides in the surface layers. The possibility of this reaction occurring is such a hazard that charges with more than 1.2 percent of carbon are only rarely employed and then only with plenty of precautions.

It would be economically attractive to be able to employ charges containing more than 1.2 percent carbon, for example 2.5 percent (although not as high as the very high carbon contents of cast irons), because it would be possible then to add to the charge the whole of the alloying elements necessary (in addition to those which are supplied as the crop ends of alloyed scrap) in the form of carburized ferro-alloys rather than the much more expensive currently used refined and overrefined ferro-alloys (these being usually employed as final additions necessitated by an insufficient chromium content in the charge). This is particularly true for refined and over-refined ferrochromes. However, this is not possible by the conventional procedures for the reasons outlined above.

The carbon imbalance between the surface and the deeper parts of the metal bath also gives rise to other disadvantages, in particular bath turbulence which gives rise to refractory wear and splashing, and scorification of the chromium, particularly at carbon levels of 0.7 percent and above. It will be appreciated, therefore, that carbon imbalance is a problem not only in that it sets a practical maximum carbon content for the charge, but also because even at carbon levels below this maximum, it gives rise to difficulties. All these disadvantages are inherent in the application of oxygen through the surface of the bath It is an object of the present invention to make possible, without significant disadvantages and without the risk of violent reaction, the employment in the electric furnace of charges having more than 0.7 percent carbon initially and, more particularly, of charges having 2 more than 1.2 percent of carbon initially, for the production of chrome steels.

There have recently been described, two methods of obtaining chrome steels of low carbon content. In the first of these, the carbon monoxide product of the decarburization of the metal bath is diluted in the bath either by means of a neutral gas or by means of steam associated with the refining oxygen. This dilution is effected once the carbon content of the bath has fallen to a critical level. This technique necessitates blowing the diluent into a converter through submerged blastpipes. One particular method employs argon.

In the second recently described method, the partial pressure of the carbon monoxide product of the decarburization, is reduced by placing under vacuum a ladle of chrome steel and blowing into the steel a jet of pure oxygen by means of a lance.

More often than not, the metal treated by these methods has first been refined in an electric furnace and hence the carbon content is limited for the reasons of safety explained above. One can of course not limit the carbon content in the mixture bed, not blow oxygen into the furnaceand merely employ the first and second methods referred to above with a high initial carbon content. But, in the case of the employment of an argon-oxygen converter, the consumption of argon becomes greatly increased and hence the refining costs are high. In the case of refinement by oxygen under vacuum, a high initial carbon content necessitates the use of very powerful vacuum pumps and the procedure is difficult in practice.

We have now found that it is possible with an electric furnace to start from any carbon content whatever in the initial charge and to decarburise easily the whole of the charge with minimum scorification of the chromium. In order to obtain very low carbon contents, one can either apply the method of the invention alone and carry out the whole of the refining in the same electric furnace (blowing in through the blast-pipe, starting at a certain stage, a fluid diluent for the carbon monoxide), or one can employ at the end of the operation at the outlet of the electric furnace, one of the known methods for obtaining very low carbon contents, for example making use of either a converter or else a ladle under vacuum furnished with a lance.

A principal advantage of the method of the invention is to be able to treat, under good technical and economic conditions, charges more highly carburized than usual and thus, by using carburized ferro-alloys, to reduce very significantly the consumption of the more expensive refined and over-refined ferro-alloys.

The invention provides a method of making a chrome steel which includes the step of blowing oxidising gas into a molten steel-makin charge steel-making chr0- mium, in an electric arc furnace to decarburise the charge, wherein the charge initially has a carbon content of above 0.7 percent, and at least one drowned jet of oxidising gas is blown into the charge below the surface thereof and passes through the bath from the bottom upwards, the or each said jet being surrounded at its periphery, at its point of entry into the charge, by a fluid refractory protectant.

Preferably, the distance from tip of the drowned blast-pipe up to the surface of the molten charge is at least 300 mm. I

The method of the invention is particularly advantageous in the hitherto difficult case of electric furnace charges containing chromium and having a carbon content higher than 1.2 percent.

The oxidizing gas employed may advantageously be pure oxygen.

The method of the present invention can constitute one particular way of carrying out the general method, as applied to chrome steels. In this, at least one jet of oxidizing gas is blown into the electric furnace by blastpipes passing through the hearth or the wall of the furnace, and each jet is surrounded at its periphery by an agent which protects the refractories against wear, this protective agent opening into the bath round each jet of oxidizing gas (which may be pure oxygen).

The present invention can also, however, be applied to oxidizing jet means of another nature, for example, of air enriched with oxygen or a mixture of oxygen and steam, protected at its periphery by an auxiliary fluid.

According to a preferred feature of the invention, the oxidizing gas is either technically pure oxygen or is oxygen containing a small quantity of a gas such as nitrogen or argon or steam, for example, in the ratio of up to 5 percent of said gas to 95 percent oxygen, the said gas having the effect of accentuating the stirring of the bath in the cases or in the phases in which the natural mixing by the carbon monoxide produced is not sufficient.

In accordance with another preferred feature of the invention, when the carbon content in the bath falls to a level at which scorification of the chromium becomes significant, the oxidizing gas blown in at depth consists of a mixture of oxygen and a fluid diluent for the carbon monoxide by-product, which diluent fluid can be, for example, argon, nitrogen, steam or water atomized into a carrier gas, in a major proportion, for example 50 to 70 percent of the total flow.

In accordance with another preferred feature of the invention, conventional blowing in of pure oxygen by lance at the surface can be combined with blowing in at depth in accordance with the invention.

At certain times duringthe decarburization, it may be found advantageous to blow in low down a mixing fluid (nitrogen, argon, steam, atomized water, etc.) and high up, by lance, the refining oxygen.

As will be understood, one of the main advantages of the method of the invention is to force the oxygen to react at depth with the carbon in the bath and so form, within the whole mass of the metal bath, bubbles of carbon monoxide which ensure good stirring of the whole bath and also between the surface metal layer and the slag.

Another advantage of the invention is a shortening of the duration of the operation, because a greater gas flow rate can be used at the bottom of the bath than can be used in top-blown lance procedures.

Another advantage of the invention is a smaller consumption of refractories, firstly because the zones of superheat are embedded in the bath, next because the splashing and spattering are less violent than with top blowing by lance, and finally because the operation is shorter.

In order that the invention may be well understood, the following Example is given by way of illustration only.

EXAMPLE A- melt of 60 tons of stainless steel containing 18 percent chromium and 9 percent nickel was made as follows.

To a 60 ton electric arc furnace was charged the following:

carburized ferro-chrome with a carbon content between 4 and 6 percent; ferro-nickel having 1.8 percent carbon; low-carbon ferro-nickel; ordinary ferro-manganese and sheet steel in bales. The melt formed analysed as follows:

C=1.7%, Si=0.570%, Mn=1.73%.

Pure oxygen was blown into the melt by means of four blast-pipes (tube 20/25 for the oxygen) fed at 12 bars at a total flow of m lminute for 11 minutes, i.e. a total volume of 1,210 m N. In these blast-pipes, each central oxygen tube was surrounded by a tube fed with fuel oil which protected the blast-pipes against wear at heat in the melt.

The analysis at the end of the blow was:

C 0.420%, Mn 1.60%, Cr 18.40%, Ni 9.1%.

Thus, there has been achieved a speed of decarburization of Ni=8.85%, Cr=18.5%,

as against 1.5 to 2 percent per hour with a conventional lance.

The loss of Cr brought was 5 kg/t of liquid steel during the blow. (From the point of view of the Cr content of the bath, the scorification of the chromium had been compensated by a reduction in weight of the bath, whence a quasi-conservation of the Cr content.)

The metal thus refined was poured into a ladle for supplementary refining by oxygen under vacuumto a carbon content of 0.012 percent (in order to limit the scorification of the chromium). The loss of Cr in this last phase was 13 kg/t of liquid steel, reduced to 4 kg/t of liquid steel by reduction of the ladle slag.

By virtue of the combination of oxidizing blast-pipes in the furnace and refinement in the ladle under vacuum, refinement was achieved of a bath having 18% Cr, from 1.7% C down to 0.012% carbon whilst consuming only 9 kg of chromium/ton of liquid steel between the end of fusion and the pouring of the metal into ingot moulds.

Due to the peripheral protection of the blast-pipes by the fuel oil,- wear of these blast-pipes is low, about 1.8 mm per operation of 11 minutes, and these blast-pipes are therefore altogether operational in an electric furnace.

As stated above, it is possible to refine to very low carbon levels in the electric furnace itself if, for example, a device for treatment under vacuum is not available, by blowing through the blast-pipes a mixture of oxygen and neutral gas such as argon, or else a mixture of oxygen and steam.

The invention is useful for the production of chrome ferrites.

In the method of the invention, the oxidizing gas is introduced into the molten metal charge from the bottom region thereof, rather than from the top as in conventional top blown electric arc furnaces. This may conveniently be achieved by introducing the gas through blast pipes which pass into the furnace through the bottom or the lower part of a wall thereof, and from which the gas is released into the bottom region of the molten metal.

I claim:

1. In a method of making a chrome steel which includes the step of blowing oxidizing gas into a molten steel-making charge containing chromium, the steps of introducing the charge in an electric arc furnace, to decarburise the charge, the charge initially having a carbon content of above 0.7 percent, blowing at least one submerged jet of oxidizing gas into the charge below the surface thereof and passing through the bath from the bottom of said furnace upwards, each said jet being surrounded at its periphery, at its point of entry into the charge, by a fluid protectant for the refractory lining of said furnace.

2. A method according to claim 1, wherein the charge initially contains more than 1.2 percent carbon.

3. A method according to claim 1, wherein the oxidising gas is introduced into the charge through at least one blast-pipe passing through the hearth or wall of the furnace.

A method according to claim 1, wherein the oxidizmg gas is selected from the group consisting of pure oxygen, air enriched with oxygen, mixtures of oxygen and a neutral gas, and mixtures of oxygen and steam, and may contain atomized water in suspension therein.

A method according to claim 1, wherein the oxidizing gas is pure oxygen and including the further step wherein, during the decarburisation, stirring of the charge is accentuated by adding to the pure oxygen 21 small quantity of another fluid selected from the group consisting of nitrogen, argon, steam and atomized water.

6. A method according to claim 5, wherein said small quantity is at most 5 percent of the total oxygen and fluid flow.

7. A method according to claim 1, wherein if the carbon content of the charge falls to a level at which significant scorification of the chromium takes place, thereafter the composition of the oxidizing gas is changed and consists of a mixture of oxygen and a fluid diluent for the carbon monoxide produced in the charge by the decarburisation, the fluid diluent constituting a major proportion of the total flow.

8. A process according to claim 7, wherein the diluent fluid is selected from the group consisting of argon, nitrogen, steam and atomized water in a carrier gas.

9. A method according to claim 1, including the further step wherein pure oxygen is blown into the surface of the bath of lance, in addition to blowing said oxidizing gas into the charge at depth.

10. A method according to claim 1, including the further steps wherein, at one or more stages during the decarburisation, pure oxygen is blown into the surface of the bath by lance and a stirring fluid is blown in at depth, the oxidizing gas flow at depth being temporarily chrome steel produced is stainless steel. 

1. IN A METHOD OF MAKING A CHROME STEEL WHICH INCLUDES THE STEP OF BLOWING OXIDIZING GAS INTO A MOLTEN STEEL-MAKING CHARGE CONTAINING CHROMIUM, THE STEPS OF INTRODUCING THE CHARGE IN AN ELECTRIC ARC FURNACE, TO DESCARBURSIE THE CHARGE THE CHARGE INITIALLY HAVING A CARBON CONTENT OF ABOVE 0.7 PERCENT, BLOWING AT LEAST ONE SUBMERGED JET OF OXIDIZING GAS INTO THE CHARGE BELOW THE SURFACE THEREOF AND PASSING THROUGH THE BATH FROM THE BOTTOM OF SAID FURNACE UPWARDS, EACH SAID JET BEING SURROUNDED AT ITS PERIPHERY, AT ITS POINT OF ENTRY INTO THE CHARGE, BY A FLUID PROTECTANT FOR THE REFRACTORY LINING OF SAID FURNACE.
 2. A method according to claim 1, wherein the charge initially contains more than 1.2 percent carbon.
 3. A method according to claim 1, wherein the oxidising gas is introduced into the charge through at least one blast-pipe passing through the hearth or wall of the furnace.
 4. A method according to claim 1, wherein the oxidizing gas is selected from the group consisting of pure oxygen, air enriched with oxygen, mixtures of oxygen and a neutral gas, and mixtures of oxygen and steam, and may contain atomIzed water in suspension therein.
 5. A method according to claim 1, wherein the oxidizing gas is pure oxygen and including the further step wherein, during the decarburisation, stirring of the charge is accentuated by adding to the pure oxygen a small quantity of another fluid selected from the group consisting of nitrogen, argon, steam and atomized water.
 6. A method according to claim 5, wherein said small quantity is at most 5 percent of the total oxygen and fluid flow.
 7. A method according to claim 1, wherein if the carbon content of the charge falls to a level at which significant scorification of the chromium takes place, thereafter the composition of the oxidizing gas is changed and consists of a mixture of oxygen and a fluid diluent for the carbon monoxide produced in the charge by the decarburisation, the fluid diluent constituting a major proportion of the total flow.
 8. A process according to claim 7, wherein the diluent fluid is selected from the group consisting of argon, nitrogen, steam and atomized water in a carrier gas.
 9. A method according to claim 1, including the further step wherein pure oxygen is blown into the surface of the bath of lance, in addition to blowing said oxidizing gas into the charge at depth.
 10. A method according to claim 1, including the further steps wherein, at one or more stages during the decarburisation, pure oxygen is blown into the surface of the bath by lance and a stirring fluid is blown in at depth, the oxidizing gas flow at depth being temporarily suspended.
 11. A method according to claim 1, wherein the fluid refractory protectant is fuel oil.
 12. A method according to claim 1, wherein the chrome steel produced is stainless steel. 